Traumatic brain injury (TBI) in children is the leading cause of death and disability. Although clinical studies have shown that the developing brain is particularly vulnerable to injury, the basis for this vulnerability remains unclear. Here we will determine if matrix metalloproteinase (MMP)-9 is a key initiator of early tissue damage and that modulation of its activity will confer early neuroprotection and establish an environment that is favorable to brain development and cognitive recovery. The gelatinase MMP-9 is a member of the MMP family of proteolytic enzymes that are secreted in an inactive form and are activated in the extracellular matrix (ECM) by a variety of mechanisms. While MMPs are critical for normal brain functioning, excessive and uncontrolled activity leads to dysregulated proteolysis, culminating in cell injury/death. Here we will determine if MMP-9 initiates an early self-perpetuating injury response that is coupled to activity of neutrophil elastase and neutrophil infiltration. We hypothesize that MMP-9 mediates early tissue damage by inactivating a1-protease inhibitor, the primary physiologic inhibitor of neutrophil elastase, thus supporting neutrophil elastase-mediated neuronal injury and furthering leukocyte recruitment. Using complimentary pharmacologic and genetic strategies, together with adoptive transfer, we will examine the cooperativity between MMP-9 and neutrophil elastase in signaling neutrophil recruitment and mediating early cell injury. Aim 1 will test the hypothesis that elevated MMP-9 contributes to disruption of the blood-brain barrier, neural injury, and white matter damage. Aim 2 will test the hypothesis that MMP-9, conveyed by infiltrating neutrophils, promotes disruption of the blood-brain barrier. Aim 3 will determine if MMP-9 inactivates a1-protease inhibitor, an inhibitor of neutrophil elastase, thus allowing neutrophil elastase to produce tissue injury and further neutrophil recruitment. Aim 4 will test the hypothesis that blockade of early gelatinase activity in the acutely injured brain will result in long-term structural and behavioral recovery. To test these hypotheses we will use a murine model of TBI at postnatal day 21 and several strategies to modulate MMP-9 and neutrophil elastase activity. We will compare early indices of tissue damage including barrier dysfunction in brain injured wildtype (WT) mice to transgenic mice with a null mutation in MMP-9 or overexpress tissue inhibitor of matrix metalloproteinase-1. The interdependency of MMP-9 and neutrophil elastase in initiating a self-perpetuating injury response will be examined using adoptive transfer techniques in WT and MMP-9 and neutrophil elastase null animals. With state-of-the-art magnetic resonance imaging and a comprehensive battery of behavioral assays, we will further determine if early pharmacologic blockage of gelatinase activity supports structural recovery and improves long-term cognitive outcomes. Together, these studies provide an important foundation for understanding the unique vulnerability of the young brain to TBI and for developing the most appropriate therapies for the brain-injured child.